This invention relates, in general, to electrophotography and, in particular, to charge generation layers for electrophotographic imaging members using a tungsten exposure.
In electrophotography, an electrophotographic imaging member containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer, while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image can then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image can then be transferred from the electrophotographic imaging member to a support, such as paper. This imaging process can be repeated many times with reusable photoconductive insulating layers.
As such photoconductive materials, inorganic materials have often been used. In electrophotographic photoreceptors, for example, inorganic photoreceptors provided with a photosensitive layer that contains selenium, zinc oxide, or cadmium sulfide as the primary component have been widely used.
However, these inorganic photoreceptors are not always satisfactory in characteristics of photosensitivity, thermal stability, moisture resistance, and durability, which are essential for electrophotographic photoreceptors used in copying machines. For example, selenium is liable to crystallize from heat or stain, creating finger spots.
For improving upon the disadvantages of inorganic photoconductive materials, various organic photoconductive materials have come to attract much attention in the art, and a number of approaches have been made to use them in the photosensitive layer of electrophotographic photoreceptors.
An electrophotographic imaging member may be provided in any of a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium, or it may be a composite layer containing a photoconductor and another material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive insulating organic compound dispersed in an electrically insulating organic resin binder.
U.S. Pat. No. 3,904,407 to Regensburger et al. discloses an electrophotographic plate having a photoreceptor comprising a photoinjecting pigment selected from the class of perylene pigments and an active transport material that is substantially transparent in the wavelength region of xerographic use and capable of supporting charge carrier injection from the pigment.
U.S. Pat. No. 4,232,102 to Horgan et al. discloses an imaging member comprising a layer of organic resin in which a photoconductive material comprising trigonal selenium is dispersed. This layer can be the charge generation layer in an imaging member also containing a charge transport layer. The photoconductive material so prepared is useful for improving cyclic charge acceptance and control, and for improving dark decay.
U.S. Pat. No. 4,578,333 to Staudenmayer et al. discloses an imaging member comprising a charge generating layer comprising a photoconductive pigment such as a perylene compound, a charge transport layer, and an acrylonitrile copolymer interlayer disposed between the charge generating layer and the support.
U.S. Pat. No. 4,587,189 to Hor et al. discloses photoconductive imaging members comprising a vacuum sublimation deposited benzimidazole perylene charge generating layer for photoelectric imaging and performance enhancement.
U.S. Pat. No. 4,639,402 to Mishra et al. discloses an imaging member comprising an organic resin binder and photoconductive materials containing selenium particles coated with a hydrolyzed amino silane.
U.S. Pat. No. 4,988,595 to Pai et al. discloses a listing of photoconductive materials including, inter alia, amorphous and trigonal selenium, phthalocyanines, dibromoanthanthrone, and benzimidazole perylene, that can be used as charge generating materials in photogenerating layers. It is further disclosed that charge generating binder layers comprising particles or layers comprising a photoconductive material such as vanadyl phthalocyanine, metal free phthalocyanine, benzimidazole perylene, amorphous selenium, trigonal selenium, selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium arsenide, and the like and mixtures thereof are especially preferred because of their sensitivity to white light. See, also, U.S. Pat. Nos. 5,089,369 and 5,164,276.
There is a continuing interest in the development of photoreceptors in which manufacture is simplified, print defects are reduced, particularly over extended use, and useful life is lengthened. Benzimidazole perylene (BzP) is a useful pigment in such photoreceptors because, in addition to being an organic pigment, it represents no health threat or other hazard to the environment, such as some other inorganic and other known carcinogenic organic pigments. Another reason for using BzP is that it is a pigment with good cyclic and environmental stability with regard to its xerographically relevant electrical properties, while simultaneously possessing adequate photosensitivity across nearly all of the visible spectrum.
Using BzP alone as the photoactive pigment results in two machine performance shortfalls. The first is the possible appearance of background print-out. Such background can be acceptably diminished by altering the xerographic setpoints of a xerographic machine, but doing so would violate the goal of designing a photoreceptor that performs equivalently to the photoreceptors having a tungsten light source that are already on the market.
The second manifestation of machine performance shortfall due to the use of BzP alone as the photoactive pigment is in the relative grey level response to various colored input document color and halftone patches. That is, for example, the relative grey level response to a blue patch compared to a yellow patch of the same value on the input document results in one ratio of output grey levels for a photoreceptor already marketed, and a different output grey level ratio for a particular photoreceptor design using BzP alone as the photoactive pigment. Owing to the shape of the spectral response inherent to BzP pigment as compared with that of the desired response, in combination with the input optics, simple design changes in the BzP based photoreceptor, such as layer thicknesses, or pigment to binder ratios, or similar such design parameters as are well known in the art, cannot provide adequately equivalent grey level response to various colored input as the desired response, while simultaneously maintaining equivalent xerographic electrical properties.